The present invention relates to an amorphous silicon photosensor.
It is required that an image sensor using amorphous silicon be as simply structured as possible by forming the elements of the sensor in the form of a thin film and making them integral in order to obtain an image sensor with a low price, but capable of attaining high image resolution and high S/N ratio.
Conventionally there are three types of amorphous silicon photosensors as shown in FIGS. 1A, 1B and 1C.
Japanese Laid-Open Patent Application 46-26478 discloses an MIS type semi-conductor photoelectric conversion device or photosensor in which an insulating layer thereof is made of a nitride. More specifically, FIG. 1A shows a schematic cross-sectional view of such an MIS type amorphous silicon photosensor. This photosensor is structured in such a configuration that a lower electrode 102, an intrinsic amorphous silicon layer 103, an insulating layer 104, and an upper transparent electrode 105 are successively overlaid on a substrate 101. In this MIS type photosensor, the insulating layer 104 interposed between the intrinsic amorphous silicon layer 103 and the upper transparent electrode 105 comprises an insulating material such as SiO.sub.2 or a nitride, whereby a rectifying function is provided. The insulating layer 104 must be as thin as 100 .ANG. or less. However, in practice, it is extremely difficult to make such a thin insulating layer with a uniform thickness.
Japanese Laid-Open Patent Application 57-106179 discloses a photoelectric conversion device having a Schottky barrier contact with a metal layer and a semi-conductor layer. More specifically, FIG. 1B shows a schematic cross-sectional view of such a Schottky barrier type amorphous silicon photosensor. This photosensor is structured in such a configuration that a lower electrode 112, an intrinsic amorphous silicon layer 113 and a Schottky barrier semi-transparent metal layer 114 are successively overlaid on a substrate 111. In this photosensor, the height of the Schottky barrier produced by the contact of the semi-transparent metal layer 114 with the intrinsic amorphous silicon layer 113 is changed by changing the magnitude of a bias voltage applied thereto. By utilizing this property, this photosensor is provided with a rectifying function. However, in this photosensor, it is considered that a thin oxide layer is usually present between the amorphous silicon layer 113 and the metal layer 114, and the dangling bonds of the amorphous silicon layer 113 are also present at the interface of the two layers 113 and 114, so that it is difficult to control the Schottky barrier at the interface of the two layers.
Japanese Laid-Open Patent Application 56-142680 discloses a PIN type photoelectric conversion device. FIG. 1C shows a schematic cross-sectional view of such a PIN type amorphous silicon photosensor. This photosensor is structured in such a configuration that a lower electrode 122, an n-type layer 123, and an intrinsic amorphous silicon layer 124, a p-type layer 125 and an upper transparent electrode 126 are successively overlaid on a substrate 121. The PIN type photoelectric conversion device can be made thinner than the other conventional photoelectric conversion devices. However, since the electric conductivity of the n-type layer and p-type layers is as large as 10.sup.-3 (S-cm.sup.-1) or more, when they are incorporated in the PIN type photoelectric conversion device, the total sheet resistance thereof is too large to ignore. The result is that each device must be spearately structured when incorporating them in an image photosensor and accordingly its manufacturing is difficult. Furthermore, in this photoelectric conversion device, since p- and n-control is performed by impurity-doping, it is not easy to make the thermal stability of the conversion device sufficiently reliable for use in practice.